The use of electronic devices has increased in many areas of everyday life. Almost in any area of the world, the space around people is immersed with the variety of emitted electro-magnetic waves. These electro-magnetic (EM) emissions vary in power and frequency occupancy. The use of the frequency spectrum is regulated by commissions delegated by governments. Some of the devices claim the spectrum as part of their intended operation like cellular phones, but some devices like computers and displays radiate unwanted emissions. These unwanted emissions compromise the operation of other electronic devices. EM emissions can cause a malfunction of devices like pacemakers or airplane navigation or landing instruments and can be life threatening.
All equipment has to pass the EM interference (EMI) requirements set by different government bodies. The Federal Communications Commission (FCC) in the United States and CISPR in Europe are regulating amounts of radiated emissions in different classes depending on the place of use. The class A regulates emissions in industrial environment and class B regulates emissions in residential environment. The class A has more relaxed specifications than class B. Another class is so called open-box equipment where equipment that is sold separately for plugging in to other devices, like in the case of many computer related products, has to meet slightly less stringent specifications than the product they are to be plugged in to.
It is mandatory to satisfy EMI requirements. If a device fails to pass EMI testing, it is very expensive to redesign the device to meet EMI requirements. Such redesign can also introduce long delays before the devices can be sold.
The high-speed digital signals in a SERDES transmitter are usually carried over PCB boards, backplanes, or cables using differential lines. These signals can contain an unwanted common mode signal that results in failing to meet the EMI (electro-magnetic interference) requirements.
The high speed SERDES transmitters usually cause EMI violations at discrete frequencies at the symbol rate of the differential signal. The main cause of non-linear common mode generation in high speed SERDES transmitter is the difference between rise and fall edges of positive and negative signals at the output, as described for example in P. Acimovic “Novel Band-Stop Mode Filter For High-Speed Digital Transmission”, DESIGNCON 2007, which is hereby incorporated by reference herein. Good engineering design can minimize the difference between rise and fall edges, but not over process-voltage-temperature (PVT) variations.
There is a substantial amount of prior art devices which use coupled inductors for the purpose of filtering the common mode signal.
The most common way of implementing the common mode filtering is the use of a common mode choke as shown in FIG. 1. The operation of the circuit in FIG. 1 is well known. A high value of magnetic coupling coefficient between L1 and L2 in the FIG. 1 circuit is essential for effective common mode signal reduction. For this reason a material with high magnetic permeability is used in transformer construction. This is necessary because the differential signal insertion loss is increased if the magnetic coupling coefficient value is below 0.95. Current state of art series common mode chokes are typically not useful above 5 Gbps. Circuits with several common mode chokes making common mode filter, as disclosed for example in U.S. Pat. No. 5,077,543, are only usable at even lower data rates.
U.S. Pat. No. 7,005,939, discloses, in FIGS. 7 and 8, essentially the same well known circuit of FIG. 1, but implemented on a silicon die. It is currently not possible to integrate CMOS circuits with high permeability magnetic materials on a silicon die, therefore there is little possibility to achieve magnetic coupling coefficient above 0.95, which would be necessary for effective filtering of common mode signal spurs. Also, the transformer disclosed U.S. Pat. No. 7,005,939 in is not perfectly balanced, which can be seen in FIG. 8 thereof. This can lead to number of signal integrity degradations and makes this approach difficult to use for data rates above 20 Gbps.
Referring to FIG. 6 of U.S. Pat. No. 7,385,466 elements 105a and 105b form a regular common mode choke. This implementation uses ferromagnetic material and the design depends on successful implementation of common mode choke that presents a high common mode impedance. The integration of ferromagnetic materials on a silicon die is not possible for current state of art CMOS integrated circuits, so this is not a useful topology in a VLSI IC design.
U.S. Pat. No. 7,728,692 discloses primary and secondary winding elements (122, 124) of a transformer element (120) (see FIGS. 3 and 4) are connected in series across the two line conductors, with common point connected to impedance Z1. This method is obviously totally different than our proposed method.
Another prior art approach is disclosed in Kim et al. (Jintae Kim, Hamid Hatamhani and Chih-Kong Ken Yang titled “An 8 Gb/s Transformer Boosted Transformer with >Vss Swing”, 2006 IEEE International Solid-State Circuits Conference, 1-4244-0079-1/06). Kim et al. discloses a circuit implementation in FIG. 4.8.2 thereof that uses a transformer in the transmitter output, but in a different configuration than our proposed circuit. The circuit of Kim et al. actually increases the common mode signal spurs due to further enhancing the difference between the rise and fall edge disparity.
The circuit of Kim et al. also cannot be used with programmable de-emphasis, as it would suffer from undershoot. This would reduce the eye opening and in some severe cases even close the eye in the signal eye diagram. This is clearly visible in their waveform and in the eye diagrams they presented (see FIG. 4.8.6 of Kim et al.) If the circuit of Kim et al. used de-emphasis, it would have a large drop of signal voltage at instances when de-emphasis circuit is active, and it would most likely close the eye. Therefore, the circuit in of Kim et al. is not useful for channels that have large insertion loss at the Nyquist frequency, which is usually the case for data rates above 10 Gbps.
The inventor has determined a need for apparatus and methods for reducing the probability of equipment failing to pass EMI testing, and in particular, for reducing problems associated with common mode signals in SERDES transmitters.